Brake Master cylinders

very boring for you big brains out there - but be kind to this mathless monkey.
Will fitting a tandem cylinder [of the same bore as the existing single cylinder] reduce pedal travel ?
my basic workings suggest that single piston displaces X volume with Y travel
tandem pistons displace 2 x X volume for Y travel , but as X cannot be increased then Y must be halved ?

No, to reduce pedal travel you will have to go to a larger MC. Or smaller wheel cylinders/ callipers whatever. Or alter the pedal leverage eg 6-1 instead of 7-1. The only advantage [and often disadvantage ] of a tandem cylinder is some brakes when a wheel cylinder fails you at least in theory have brakes still on one end of the car. I have found on a couple of occasions this is not always true! And lose fronts and have some rears is not always a better proposition. Losing rear and retaing fronts though is ALWAYS better than none.

Manufactures normally use a larger bore cylinder when installing a master vacc. Some older models used an inline booster on fronts only and rear were unassisted, usually drum rear. This actually is a very good idea as you get less ' compressability' of the pedal boosting only fronts. meaning a firmer pedal. Though modern ones usually work fairly well and seldom have these problems. Usually coupled with staggered bore master cylinders.

And bleeding can on occasion be a real drama.It should not be but often it is. Even just a simple flush the fluid through can cause problems.

The manufacturess seem to go out of their way to make something quite simple very sophisticated.
ABS brakes also often adds to these problems. I have worked on a couple of mainstream models that this is always a drama on, even when changing pads.

But I don't understand 'why' a tandem cylinder doesn't displace more fluid for a given bore/stroke ratio over a single piston

the primary piston displaces fluid to eg the front half the system , the secondary piston displaces fluid to the other, rear half the system
so each piston only has to displace half the given volume over a similar bore single piston cylinder for the given braking system
therefore half the piston stroke
?

But I don't understand 'why' a tandem cylinder doesn't displace more fluid for a given bore/stroke ratio over a single piston

the primary piston displaces fluid to eg the front half the system , the secondary piston displaces fluid to the other, rear half the systemso each piston only has to displace half the given volume over a similar bore single piston cylinder for the given braking system therefore half the piston stroke ?

I would be inclined to agree - you would think that a tandem MC amounts to two separate MC's.

I have lost the rear brakes on a couple of occasions and kept the front discs - unless you are braking very hard it is difficult to notice any difference in the stopping power.

Which makes wonder - on a very light car (or motorcycle) do you need rear brakes at all? You see MotoGP riders braking hard with the rear wheel off the ground and the same with some lightweight racing Minis - so the rears don't do much. I have also seen advice from experienced 'bike racers to beginners in racing to never use the rear brake.

I don't think the original poster was envisioning a split system, rather two cylinders feeding one system. Pedal travel would be reduced if two cylinders each equal in diameter to the previous single cylinder fed the same volume of calipers.

Yes, I think there lays the "problem". Carlt - I think (but maybe I'm wrong) you envision it as two pistons in parallel, but in a conventional tandem cylinder, this is not the case.Just to be sure, that we are all on the same page, is this what you call a "tandem" cylinder?

The movement of the individual pistons is defined my the stiffness of the individual circuits.

In other words, if we assume equal stiffness, each piston of the Tandemcylinder would only be displaced by 1/2 the travel,keeping the overall travel the same.

your tandem cylinder is the same as my tandem cylinder
the 'other' system I know as a dual circuit system with balance bar , as generally used in competition cars

If we use your diagram above - that is showing brakes on / pedal fully depressed.
Brakes off is pushrod back the length of green arrow / primary and secondary piston cup seals back to reservoir inlets .
So the stroke is the length of the green arrow - this stroke is required to displace enough fluid to operate the front and rear slave cylinders

Lets turn this cylinder into a single piston one - remove the primary piston and fit a longer pushrod that operates the secondary piston . blank off the primary inlet and outlet , tee the primary circuit into the secondary outlet - [a long winded hypothetical way of fitting a single piston master cylinder of the same bore]
So now when we depress the pushrod by the same stroke as the green arrow we are only moving the secondary piston , which is only going to displace the same amount of fluid as it did before , this is only enough to operate the rear slave cylinders -

For a single cylinder to displace the equivalent volume as your tandem cylinder above the single piston will have to move 2 X the green arrow
?

Good, now we know that we are on the same page.But, I think, you still assume that the two pistons are "solid" coupled - which they are not.If they where, how would the "left" piston (in respect to the drawing above) displace any fluid? The distance between the first (left) and second (right) piston would stay the same ( if we assume a solid link), therefore no fluid would get displaced out of the cylinder and into the brake circuit.The fluid volume, between the two pistons would just move to the right in the drawing.To displace the fluid, you need a relative movement between the left and right piston, making the volume between them smaller -> forcing the fluid out of the cylinder.

This "problem" is equivalent to trying to compress two springs with stiffness k which stand on top of each other (in series) compared to compressing a single spring with stiffness k/2. In both cases the total deflection is the same for a applied force.

you are welcomethink about the second piston as a floating piston, then it becomes clear, as you have already discovered now.the primary circuit does not "build up pressure" before, but maybe take the slack up first, depending what the resistance and friction difference is.the fluid will go, where it is easiest to go, when pressure would start to build the floating piston will move until both circuits "are ready" to build pressure.the overall pressure in the system will be the same, unless the springs shown (in the drawing in the first post) have different stiffness, to achieve a pressure difference. , or the second piston has different "active" areas,exposed to the fluid in both circuits.

pretty ordinary sketch - apologize, but in it's most simple form, the difference would boil down to this.

a simplified way to think about it, is that in the tandem cylinder only one piston moves at any one time to displace volume.

can we be absolutely sure it is a seamless transmission from primary to secondary pistons though ?

Yes. The secondary piston is only there to allow one circuit to be pressurised if the other circuit leaks.(Assuming the cylinder is the same bore in both sections) the pressure is the same both sides of the secondary piston UNLESS there is a leak in one of the circuits.If there is a leak, then the pedal will travel until the mechanical travel is used up in whichever part section of the cylinder is connected to the leak and then the pressure increases in the other section.

- a standard tandem cylinder works in the way described, with the second piston being a "floating" separation piston to keep the two circuits separate,in case of fluid/pressure loss in one circuit.- in practice it looks more like the one shown on the left- the "extensions" are there to limit the possible "dead travel" of the pedal, as you don't want to run out of pedal travel (hitting the floor, bulkhead or pedal stop) if one circuit fails.- if one circuit loses fluid, or the stiffness in the circuit reduces dramatically, due to boiling fluid, are aeration of the fluid, the piston "powering" this circuit, can only travel a set amount (dead travel - usually 1/2 of pedal travel), before it will hit it's stop, then the remaining pedal travel will/can be used to pressurize the still functioning circuit.

- now, what you have in mind carlt, I think looks like what I tried to sketch in the center. The pedal would move both/two pistons simultaneously. Notice that I have drawn a wall, to separated the cylinder into two chambers. This avoids the problem described before, and allows both pistons to displacefluid at the same time. It's possible to design/build such a system - no problem, but it's not what a "normal" tandem brake cylinder does.

If we look at it a bit more closely, you will notice something else. Now the pedal moves two pistons at the same time, displacing twice the fluid, as you envisioned/intended, but you have to keep one thing in mind.P = F/A --> Pressure = Force / AreaAs you now move two pistons at the same time, you have effectively doubled the area (* see below). If you still want to generate the same pressure, now you need twice the force to achieve this.No free cheese here . Yes with such a system, you would halve your pedal travel, but you "pay" for this with twice the force needed.Technically, from a force vs. area vs. pressure point of view, your hypothetical system is the same as shown on the right, where you move two pistons at the same time via one pedal. This is a "parallel" system as commonly used with a balance bar layout. You just tried to "package" it in line.

(*) as this is the technical forum, let's be a bit more precise. In the middle drawing, Piston A is connected to Piston B via a rod, even if small in diameter/area, this means that Piston A has a slightly smaller "active" area then Piston B. This mens for a given force, it would generate a slightly higher pressure. This means that the system in the middle and on the right, are not totally identical, but the underlying principle is still the same, and what was said is true in principle.

- a standard tandem cylinder works in the way described, with the second piston being a "floating" separation piston to keep the two circuits separate,in case of fluid/pressure loss in one circuit.- in practice it looks more like the one shown on the left- the "extensions" are there to limit the possible "dead travel" of the pedal, as you don't want to run out of pedal travel (hitting the floor, bulkhead or pedal stop) if one circuit fails.- if one circuit loses fluid, or the stiffness in the circuit reduces dramatically, due to boiling fluid, are aeration of the fluid, the piston "powering" this circuit, can only travel a set amount (dead travel - usually 1/2 of pedal travel), before it will hit it's stop, then the remaining pedal travel will/can be used to pressurize the still functioning circuit.

- now, what you have in mind carlt, I think looks like what I tried to sketch in the center. The pedal would move both/two pistons simultaneously. Notice that I have drawn a wall, to separated the cylinder into two chambers. This avoids the problem described before, and allows both pistons to displacefluid at the same time. It's possible to design/build such a system - no problem, but it's not what a "normal" tandem brake cylinder does.

If we look at it a bit more closely, you will notice something else. Now the pedal moves two pistons at the same time, displacing twice the fluid, as you envisioned/intended, but you have to keep one thing in mind.P = F/A --> Pressure = Force / AreaAs you now move two pistons at the same time, you have effectively doubled the area (* see below). If you still want to generate the same pressure, now you need twice the force to achieve this.No free cheese here . Yes with such a system, you would halve your pedal travel, but you "pay" for this with twice the force needed.Technically, from a force vs. area vs. pressure point of view, your hypothetical system is the same as shown on the right, where you move two pistons at the same time via one pedal. This is a "parallel" system as commonly used with a balance bar layout. You just tried to "package" it in line.

exactly so -the reason this all started going round in my head was the single master cylinder system i am trying to get working a little better [too much travel and force needed]with my assumption that the Tandem cylinder operated as per your middle diagram , I was hoping to fit a Tandem cylinder with smaller bore and not increase the travelAnyway- the original system work ok[just] so is staying for the time being

the reason this all started going round in my head was the single master cylinder system i am trying to get working a little better [too much travel and force needed]

If both travel and force are at the same time too large and the system is working more or less as designed with pad-rotor clearances within spec, I can't see any simple replumbing of the hydraulics being helpful for correcting that. Or am I missing something obvious?

If both travel and force are at the same time too large and the system is working more or less as designed with pad-rotor clearances within spec, I can't see any simple replumbing of the hydraulics being helpful for correcting that. Or am I missing something obvious?

my misconception for the Tandem cylinder and neglecting to always consider the system as a whole lead to the replumbing scenario
[I wouldn't say the system was ever designed - more put together from parts off 3 different cars in the '70s]
the constraints of the footbox size ,by the engine position , make it virtually impossible to alter pedal position or ratio's

forgot to mention it is all drum brakes and Any shoe drag makes the car impossible to turn [on greasy mud/clay] hence the excess pedal travel

larger Master cylinder = more force less travelin my case the brake type on the hubs will need changing - but that's an off season job

I was going to ask, what causes the "excess" travel, maybe some modifications in the periphery could lead to improvements.Now, you have answered this already, and as you see for yourself, it's difficult to achieve both objectives at he same time -> less travel & less force.

I don't know your car, and the circumstances, so just going to throw out a theoretical possibility. You will know if this is feasible or not for you(r car) and your application.

You could move to a larger diameter piston MC (or twin pistons) to reduce the travel, and then us a "booster" to account for the increased force needed.Not sure if this is better/cheaper/easier, then to convert all axles from drum to disc brakes - you decide.

I was looking for a photo from a Alfa Romeo 155 Class II (Super Touring Car), but I couldn't find one in a hurry.They used a similar layout (at least in the Spanish Championship with G. Francia in 1995), and used a electrical driven vacuum pump to power the boosters.

Not sure, if such a system would make sense, make the job easier, for you, just wanted to mention the possibility.Your call - good luck in any case

to give some idea of packaging constraints -
the clutch pedal is set back towards the driver by 6-8" to allow clearance for the oil filter on the engine - the oil filter is very narrow [about the width of a sandwich plate take off for remote filter]
the brake pedal is mounted set back from throttle pedal so foot just touches brake pedal at WOT , hence a bit of a reach on long brake pedal travel . [there is not enough room to get foot on throttle past brake pedal [which itself is only 3/4" wide]
oh yeh - the steering column passes down in the foot box between clutch and brake [a suitable guide for the foot so it doesn't miss the brake pedal and get tangled in something else
its immensely fun to drive !

the only place for something that size would be in front of the radiator !might even struggle to squeeze it in there -

Seems like your only option is to minimise the volume of fluid needed to achieve a certain braking result and then use a smaller diameter master cylinder. If everything in the system is as good as possible, ie perfectly round drums, linings bedded in, adjusters just so, best possible flexibles, perfectly bled, new fluid, master cylinder and seals set so they only just clear the reservoir port, almost zero pedal play.....

Servos cover a multitude of these little deficiencies and we are used to the way they isolate from brake feel. Recall that forty years or so ago many quite fast cars did without servos (but it was neccessary to keep right on top of brake maintenance.)And have a strong leg.

the only place for something that size would be in front of the radiator !might even struggle to squeeze it in there -

Interesting.

Just a couple of weeks ago I was peering around an Evante (sp?) and remarked to the owner my surprise that the booster/MC was ... beside the radiator. It was just about the only place for it in the engine bay.

Having only glimpsed this since my first post if you do not wish to use a booster, or lack space make a brake pedal that reverses the leverage, eg above the fullcrum than the master can be mounted in the dash area. If you use a longer lever you can dispense with the booster, or the same leverage with a smaller bore M/C. Or just find a staggered bore M/C and boost the fronts only. Willwood, US Brake make these and are not real dear. If you stay eg 1" bore and use 7/8 rear and then use an inline booster on the fronts only. 7/8-3/4 will just about do away with the booster all together though pedal effort will be firmer. An inline adjustable proportioning valve will then give fine balance.
The car I am playing with ATM uses an 1" 1.2" bore master on the factory booster. Though that has big 4 spots front and large [front] single piston callipers rear on 13" rotors. With the adjustable proportioning valve. Though with that M/C it hardly needs it now.
When you start on this sort of stuff though buy yourself a flare tool and some bundy and practice first too!

For any motorport application inline residual valves that hold 3-5 lb will keep the pedal up. And really should be used with any drum rear set up or the pedal will always be crap. All factory drum brake set ups use one, sometimes as a seperate piece but often just as the seat in the side of the cylinder. These can fail and cause the pedal to get VERY firm and the brakes drag or conversly not hold pressure and have a crap pedal like without

For a race car though it is easier and these days quite cheap to buy a complete balance bar with cylinders, again Willwood etc make these

As for a tandem cylinder displacing more fluid, it will not for the same effort. Though I guess it has the capacity too.

for this reason, I proposed a remote booster setup.As long as you have room for a MC, as you have now, it should be o.k.

Here is a principle schematic of such a setup

Was used on some Alfa Romeo road cars in the 60-70's, such as the round tail spider

here you can see a historic car, with two boosters in the back trunk.

Historic? It is a wheel tubbed LC Torana imitation drag car that is very WRONG. The tank is 250mm higher than the road car, the boosters!! are 12 feet from the front brakes. Always keep them close to the M/C, the closer the better and about 3 feet maximum.

Having only glimpsed this since my first post if you do not wish to use a booster, or lack space make a brake pedal that reverses the leverage, eg above the fullcrum than the master can be mounted in the dash area.

all good stuff thanks - but the carbs are in the dash area !there really is no more space the passenger can only fit one leg in the footbox

I have a solution to the problem with a pedal re-mount/design , but it is not the 'quick fix' I was after - so closed season work

very boring for you big brains out there - but be kind to this mathless monkey.Will fitting a tandem cylinder [of the same bore as the existing single cylinder] reduce pedal travel ?my basic workings suggest that single piston displaces X volume with Y travel tandem pistons displace 2 x X volume for Y travel , but as X cannot be increased then Y must be halved ?

Remember that the MC piston linear travel is not the same as the linear travel at the pedal face. It is a ratio. With a tandem MC, assuming the pistons have equal travel, the pedal will only require a linear travel of half as much to displace the same fluid volume. However, since the pedal force is being applied to twice the piston area, the resulting system pressure will only be half as great.

Manual hydraulic brake systems are essentially hydrostatic. This means that the force equilibrium in the system is simply P/A, disregarding any fluid effects and structural elasticities.

I have to admit that for quite a while I could not see why the pedal travel would not be halved with a tandem MC - but as TC3000 (et al) says - the pedal/pushrod travel for the tandem MC is the same as for a single cylinder of the same diameter.

The way I express the situation in words is: assume the single cylinder pushrod travel is one inch for example (it may only be a couple of mm but it doesn't matter). The front piston (furthest from the pushrod) does actually move only half an inch - but as the rear piston acts on the front piston, while the rear piston is also moving forward half an inch - the front piston is moving away from the rear piston by half an inch - so the travel of the pushrod/rear piston is actually one inch. The travel on the front piston is half an inch. And the travel on the rear piston relative to the front piston is also half an inch - so I suppose you could say that the piston travel is halved - but the pushrod "sees" its travel as one inch.

One thing I really envy about you islanders, you can have fun with your cars. Around here either you have a certified antique or a $200K car to be accepted in a club and get track days or you have to go James Dean, and that's not cool, right?

I'd never seen such a thing - didn't know they existed. Looks like fun.

its an early 70's version , but essentially the sameoriginally built with a Morris minor engine and box - but converted mid 70's to Renault 16TX[reversed in the chassis] coupled to a shortened Ford Anglia 105e gearbox driving through an Austin A60 back axlecheap as chips , but as you say loads of fun

the modern sporting trials cars are very sophisticated bits of kit - Hewland transaxles , single F1 type coilover on the rear , costing £30,000 ish

These used to be quite popular here in Oz in the past. And so many combos depending on the type of event. Some were quite fast and some slow. Clem Smith, the Mallala owner had one with an alloy slant 6 Valiant!
Though others were powered by very simple engines like sidevalve Anglia engines. I think a few still play even now.